I read your message and my prayers are with you. Our family is suffering from the same pain. Our mother has been diagnosed and we do not want to tell her. She is 82 and we sense that she would be so frightened of the future that her last days would not be happy.
When we have to make decisions that effect someone else the process is very difficult.
We are told that death is part of nature and we must accept it with grace. Why is this so painful.
Keep trying to make her happy and remind her that you love her. She will be a very happy lady if she knows this.
Please know that you have done the right thing just by being there.